German patent application DE 10 2009 032 286 A1 describes a planetary differential having a planetary gear system of the generic type. The planetary differential has two planetary steps, each of which is made up of a set of planets and a sun. The suns and the planets are gearwheels with face gears which are configured as involute gears. The planetary gearwheels are rotatably mounted on planet pins so that they can rotate around the sun at a radial distance from the main axis of the planetary differential and their teeth mesh with the teeth of the sun gearwheel.
The planetary pins are affixed to a planet carrier. The teeth of the planetary gearwheels of one set and the teeth of each sun wheel belonging to the planetary step mesh in such a way that, in each of the meshing points, at least one first tooth on a first toothing consisting of first teeth distributed around the circumference positively engages into a tooth gap of a second toothing consisting of second teeth. In this process, a first flank of the first tooth touches at least a second flank of a second tooth that, together with another second tooth, delimits the tooth gap in the circumferential direction of the gearwheel. The teeth touch each other at the point of tooth contact, as is normally the case with face gears. “Meshing of the teeth” refers here to the positive movable connection of the toothing of a gearwheel to a mating gearwheel through the reciprocal meshing of teeth of the gearwheel into tooth gaps of the mating gearwheel and vice versa.
Such planetary differentials are highly suitable for installation into transfer gears. Moreover, the planetary sets, planetary gear systems and planetary differentials are extremely well-suited for use in drive units powered by electric motors and employed for hybrid drive trains. Such a drive unit is described in German patent application DE 10 2008 061 946 A1. The drive unit has a main drive and a secondary drive. The main drive is connected by gears to a planetary differential via a planetary set. A planetary differential is essentially characterized by two sets of pinion gears that are each formed by a set of planetary gears. Each of these pinion gears is mounted so that it can rotate on a planetary pin around an axis of rotation that corresponds to the axis of symmetry of the planetary pin. The axes of rotation of the planet gears are oriented parallel to the axis of rotation of the driven wheels, in other words, parallel to the axes of rotation of the sun wheels of the planetary differential. The coaxial axes of rotation of the sun gears are concentric to the differential and lie on the main axis of the drive unit. The main axis of the drive unit also combines the axes of rotation of the drive shafts of the main and secondary drives. The differential shafts of the planetary differential are sun wheels that are each connected, for example, to a driven shaft that leads to a wheel of a vehicle. The teeth of each of the sun wheels mesh with the teeth of one of the sets of pinion planetary gears.
By means of the secondary drive, a superimposed gear can be employed to additionally apply torques into the planetary differential and to influence the distribution to the differential wheels. In this case, the main drive and the secondary drive are electric motors that are arranged coaxially with respect to each other. The superimposed gear is formed by three planetary drives coupled to each other. Such drive units can be used either independently of other drive sources, for example, independently of internal combustion engines, or else together with them.
In the generic planetary differentials described in German patent application 10 2009 032 286 A1 and employed in a generic drive unit according to German patent application DE 10 2008 061 946 A1, the toothing of the first planetary gearwheels meshes with the toothing of the second planetary gearwheels. Here, the number of teeth of the first planetary gearwheels preferably matches the number of teeth of the toothing of the second planetary gearwheels, but it can also be different. At the same time, the toothing of the planetary gearwheels of one of the sets of planetary gearwheels meshes with the toothing of only one sun gearwheel, without the toothing of the planetary gearwheels of this set meshing with the toothing of the other sun gearwheel. In order for this to be possible, the planetary gearwheels of one set have to be “longer”, that is to say, axially wider than the teeth of the planetary gearwheels of the other set, and they have to axially overlap beyond the other sun gearwheel without touching it.